Thermal diodes, thermal transistors, thermal memory element and similar thermal analogues of electronic devices have been topic of theoretical, as well as experimental, works. While earlier research has been on conduction (phonon) based devices, more recent studies have been focusing on radiation (photon) based thermal rectifiers. Thermal rectification has numerous applications in thermal management, thermal logic gates and information processing.
Analogous to electrical diodes, thermal diodes are rectification devices where the magnitude of heat flux strongly depends on the sign of applied temperature bias. To quantify rectification, we employ the widely used definition of rectification ratio, i.e., R=(Qf−Qr)/Qr where Qf and Qr refer to forward and reverse heat flux, respectively. Alternatively, rectification coefficient can be defined as η=(Qf−Q96/max(Q96,Qf). There are numerous studies pertaining to near-field and far-field thermal radiation based rectification devices that exploit temperature dependent properties of a phase change materials such as vanadium dioxide (VO2) and La0.7Ca0.15Sr0.15MnO3 (LCSMO). A number of studies deal with far-field thermal radiation while several others focus on modulation of radiative heat transfer in the near-field regime. Ben-Abdallah and Biehs introduced a VO2 based simple far-field radiative thermal diode, while Prod'homme, et al., proposed a far-field thermal transistor that uses a VO2 base between a blackbody collector and a blackbody emitter. Zhu, et al., showed that temperature dependent optical properties of SiC can be used to attain negative differential conductance. Van Zwol, et al., proposed that one can take advantage of the phase transition from crystalline to amorphous state in AIST (an alloy of Ag, In, Sb, and Te) driven by a current pulse to obtain a large contrast in heat flux. In far-field limit, rectification is due to the change in emissive properties of a phase change material. In near-field limit, the difference in the coupling strength of polaritons or tunneling of surface waves between structures leads to thermal rectification. In general, it is observed that a higher rectification can be achieved in the near-field regime than in the far-field. However, it challenges persisted in developing such devices that can operate on the principle of near-field radiative transfer.
Spectral control has been studied to affect radiative heat transfer in both the far-field as well as near-field. Customization of absorption/emission spectra is often achieved by the use of multilayer thin film structures, nanoparticles, dielectric mixtures, photonic crystals, 1-D/2-D gratings and metamaterials. Absorbers that utilize Fabry-Perot cavities, Salibury screens and Jaumann absorbers and ultra-thin lossy thin films bounded by transparent substrate and superstate have been investigated for decades. Quite notably, Nefzaoui, et al., proposed using multilayer structures consisting of thin films (e.g., Si, HDSi and gold) to obtain thermal rectification. Kats, el al., have theoretically and experimentally demonstrated that a thin-film of VO2 on sapphire shows strong modulation of absorbance upon phase transition, particularly, at wavelength of 11.6 μm. Taylor, et al., recently proposed an emitter consisting a dielectric spacer between VO2 film and a reflecting substrate to achieve dynamic radiative cooling upon phase transition of VO2. Fabry-Perot resonance was achieved at 10 μm wavelength. As discussed later, we show that, by tuning the resonance at right wavelength, maximum rectification can be achieved in the proposed design.
VO2 has often been used in thermal rectification devices, because its phase-change from an insulator to a metal can be switched reversibly within a short time (˜100 fs). The common devices use either a bulk VO2 solid or its thin-film form. However, a need persists for devices which employ a VO2 based far-field thermal rectification device with a simple multilayer structure with a record rectification factor of greater than 11(η>0.91).